Images to be delivered on digital broadcasts, those to be stored in media such as DVDs and hard disks, and the like are compressed by various coding systems. The object for such compressions is to avoid constraint on a transmission band, increase the transmission speed, decrease the memory size, or the like.
There are various standards for image coding system, such as MPEG2 and H.264. And there is a case where the coding system is converted for the purpose of reducing the amount of codes in a coded image that is inputted, or the like. A transcoder once decodes the inputted coded image. Then, the transcoder encodes the decoded image by a different coding system (or the same coding system) again. Thus, the transcoder controls a bit rate of an output stream.
The following Patent Document 1 relates to a transcoder for converting an input stream compressed by a first compressive coding system into an output stream compressed by a second compressive coding system. This transcoder acquires the degree of dispersion of an image and the degree of variation of the image in a time direction during the decoding of the input stream compressed by the first compressive coding system. By using these values as evaluation values, the transcoder calculates a quantization step value to be used for the second compressive coding.
[Patent Document 1] Japanese Patent Application Laid Open Gazette No. 2008-42426
The coded image conversion technique disclosed in Patent Document 1 aims to obtain an image of high quality from the output stream while controlling the bit rate to be a target value. In other words, a method of simply controlling the bit rate to be constant regardless of the content of an image has a possibility of causing degradation of image quality due to uniform bit allocation. By adopting the method disclosed in Patent Document 1, a large number of bits are allocated to an image which needs more bits to be allocated thereto and no superfluous bits are allocated to another image which does not need more bits to be allocated thereto.
Thus, in the technique for calculating an optimal quantization step value on the basis of information on the image obtained from the input stream, the optimal bit allocation is basically predicted from the condition of the input stream. Therefore, it can be expected to achieve further optimization if the condition of the image of the actually generated output stream is also taken into consideration.